Since a semiconductor device is an electronic device extremely small in size and which is produced of accumulated semiconductor layers, the semiconductor device is unexpectedly accompanied by one or more parasitic capacities.
A parasitic capacity which accompanies a capacitor produced in a semiconductor device will be described below:
Referring to FIG. 1, a capacitor is produced of a combination of (a) an N.sup.+ -silicon layer 6 diffused in an N-silicon layer 5 which is grown on a P-silicon substrate 3 and which is isolated by P-silicon regions 4, (b) a silicondioxide film 2 produced on the N.sup.+ -silicon layer 6 and (c) a metal plate 1 produced on the silicondioxide film 2. The capacitor realizes a capacity C.sub.o between terminals A and B and parasitic capacity C.sub.s between the N-silicon layer 5 and the P-silicon substrate 3 or the earth. The equivalent circuit diagram of the above described semiconductor device is illustrated in FIG. 2.
The adverse effects of a parasitic capacity is itemized below:
1. Firstly, since a parasitic capacity is a type of junction capacity, the amount is determined depending on the amount of bias voltage applied across the junction. This means that the amount of a parasitic capacity fluctuates, following the amount of bias voltage applied across the corresponding junction.
2. Secondly, since a semiconductor device is inherently a group of accumulated semiconductor layers, the amount of parasitic capacity is large enough to garble the circuit constants of a circuit produced in a semiconductor device.
3. Thirdly, it is not easy to anticipate the accurate amount of a parasitic capacity.
4. Fourthly, the adverse effects of a parasitic capacity is extremely serious particularly for a circuit having a pair of branches of which the amount of impedance is identical to each other and each of which is connected to each other through a capacitor (hereinafter referred to as a balanced circuit), e.g. a differential amplifier, a phase shifter et al. This is because one branch alone of the balanced circuit is accompanied by a parasitic capacity C.sub.s, as illustrated in FIG. 3, resultantly garbling the impedance of one of the branches. Referring to the drawing which illustrates a differential amplifier consisting of resistors R.sub.1 and R.sub.2, a capacitor C.sub.o, transistors Q.sub.1 and Q.sub.2, a load resistor L and a constant current power supply PS, the first branch consisting of the resistor R.sub.1 and the transistor Q.sub.1 is accompanied by a parasitic capacity C.sub.s, although the second branch consisting of the resistor R.sub.2 and the transistor Q.sub.2 is not accompanied by a parasitic capacity C.sub.s. As a result, the transistors Q.sub.1 and Q.sub.2 can not be applied a balanced set of input signals, resulting in a garbled characteristic of the differential amplifier. Accordingly, a parasitic capacity readily causes an unexpected garble for the characteristics of a functional device employing any type of balanced circuits.